Managing Life Extension for LargeRotating Plant in Power Stations

Wolfgang Hahn

Abstract Risk and reliability management of rotating plant in power stations underlife extension have become a major challenge for industry. The increasing numbersof catastrophic failures are calling for a more rigorous regime to address integritythrough qualitative as well as quantitative methods. Current condition basedassessments as well as condition monitoring techniques need further developmentinto the future to assist power generators manage into unknown territory during lifeextension.

Keywords Life extension � Rotating plant � Risk � Reliability � Turbines �Cracking

1 Introduction

A Large number of the Power Stations in the United Kingdom and Europe arefacing challenges with underwriting large rotating plant integrity on plants whichhave already doubled the expected design life in running hours and number ofstarts. The importance of safe reliable plant can not be over-stressed, specifically onthe large rotating machinery such as steam turbines at risk of catastrophic failure.Whilst this risk is real and with an increasing number of turbine rotors failing inservice, industry continue to extend the life of rotors into the future. Management ofthe risk therefore requires careful consideration in which best available techniquesto use. A combination of qualitative and quantitative risk management techniquesare needed to demonstrate the risk mitigation during the life extension process.

Risks related to turbine bore cracking, shaft cracking and blade cracking are casestudies covered in this paper during life extension.

W. Hahn (&)EDF Energy, West Burton Power Station, Retford, Nottinghamshire DN22 9BL, UKe-mail: wolfgang.hahn@edfenergy.com

© Springer International Publishing Switzerland 2015J.K. Sinha (ed.), Vibration Engineering and Technology of Machinery,Mechanisms and Machine Science 23, DOI 10.1007/978-3-319-09918-7_3

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2 Theory Related to Life Extension

A holistic approach to life extension is required considering all components andinfluencing variables that may contribute to the overall life expectancy of criticalcomponent behavior as indicated in Fig. 1. Plant long life is a term introduced byRiddell referring to the investment costs for plant wanting to extend the operatinglife further. This is a very important consideration for power plants as investment inlife extension may only cost around one third of a new built power station of thesame capacity.

2.1 Reliability

Risk and reliability related to failure was demonstrated in the asset managementstrategy as published by Hahn [1], the Gaussian distribution of the failures relatedirectly back to the number of like for like components exposed to similar variableconditions within a margin of tolerance. The six basic accumulative influences ofplant degradation mechanisms are creep, fatigue, corrosion, erosion and vibrationwith a combination of any of these.

Risk and reliability form a unique combination in the top event probability offailure with consequential outcome. This assessment can normally be seen in thequalitative approach used in the form of a five by five matrix; however, for com-ponents life predictions coupled to changing conditions associated with one or more

Life extension: Main technical elements for consideration on long life.

Operational influences on plant life

Plant materials behaviour during life extension to end of life (Materials life management program)

Risks types related to plant life extension

Reliability of strategic components including vertical and lateral layers of the plant during life extension to end of life.

Strategic retrofits for full life extension and performance enhancement

Reliability of the plant

Risk of plant life extension

Strategic knowledge and information management for life extension.

Long life costs and strategic investment plan for life extension

Requirements between 2011 and 2023

Underpinned &

underwritten safety case

Operational sustainability optim

isationSuccessful asset m

anagement program

Long life program up to and beyond de-com

missioning

Successful ageing managem

ent program &

model

Life extension: Main technical elements for consideration on long life.

Operational influences on plant life

Plant materials behaviour during life extension to end of life (Materials life management program)

Risks types related to plant life extension

Reliability of strategic components including vertical and lateral layers of the plant during life extension to end of life.

Strategic retrofits for full life extension and performance enhancement

Reliability of the plant

Risk of plant life extension

Strategic knowledge and information management for life extension.

Long life costs and strategic investment plan for life extension

Requirements between 2011 and 2023

Underpinned &

underwritten safety case

Operational sustainability optim

isationSuccessful asset m

anagement program

Long life program up to and beyond de-com

missioning

Successful ageing managem

ent program &

model

Fig. 1 Plant life extension considerations for long life during plant lifecycles

40 W. Hahn

variables such as temperature, pressure and/or fluid behaviour, this approach aloneis unacceptably coarse.

Figure 2 reflects an approach more closely coupled with the quantitative,qualitative and related sensitivity factor for the associated operational impacts of theplant. The original two stream approach from Barringer [2], has been improved bythe author to a three stream approach fusing data using probability statistics, in thiscase using performance effectiveness index, PEI as an independently defined out-come from system expediency [3].

Accurate taxonomy of the plant and systems is of utmost importance for real-istically addressing the components at risk as well as classification of componentsfor laboratory testing and validation of deterioration and remaining life left. Levels1–5 and levels 6–9 as defined by British Standard 14224, defines the high levelcategorization related to the industries and plant applications as well as the lowlevel related to the equipment, unit and lower indenture levels respectively. Theselower levels become more applicable in the parent to child relationship for reli-ability in the sub units and components such as steam turbines [4].

An example of criticality selection in the turbine island taxonomy process isreflected in Fig. 3. This reflects back on large rotating plant in terms of a qualitativeand quantitative approach for equipment selection. Again, in each case the com-ponent history, laboratory testing results and modeling results must support con-fidence levels for life assessments. There is nothing linking true differentiationbetween the layers from a quantitative perspective in Fig. 2. The true differentiationin a taxonomy process is demonstrated in the criticality ranking of the componentand its failure mode impact on the plant.

The process of using risk and reliability as referred to in Fig. 1 is now com-plimented using Figs. 2 and 3 respectively, building the risk register by area andpopulating the fused data according to the risk ranking for taxonomy in a risk log.

Risk classification using FMECA

Qualitative process Quantitative process

List systems and unitsseverity

levels of probability in a ranking of

importance.

List and complete systems and units

failure mode and rate distributions

impact, up to 10e-5.

Calculate criticality numberCalculate risk priority number

Rank criticality in descending order Rank RPN in descending order

Data fusion & Mapping of results into a matrix

Identify limit of risk tolerability items and list actions.

Accumulated Operational damage

List Fleet Critical Components

Taxonomy process

List and complete systems and units

accumulated damage from operational

data (normal & excursion)

Calculate PEI

Rank PEI in descending order

Fig. 2 Failure mode and effects criticality analysis for major components

Managing Life Extension for Large Rotating Plant… 41

2.2 Differentiation Between Life Extension and Wear Out

Plants running in the wear out phase have been well defined and investigated as perprevious paper produced by Hahn [1], however a plant running the life extensionperiod, phase 4, may have different variables defined for changing the wear out rateand thus influencing the hazard phase curve. An example of the bathtub curverepresents the hazard phase plot of the plants through the whole lifecycle phase asper Fig. 4. The hazard plot, Z (t), can thus be influenced at point A, when lifeextension is defined and announced on a unit, sub-unit or component level and thenecessary operational, maintenance and engineering integrity policies incorporatedinto the life management programme.

The Gaussian distribution for the items under consideration for life extensionneeds to meet the beta value in order to qualify for the life extension managementprogramme.

This means that the following rules would apply:

1. Beta < 1: Running in failure (no life extension possible)—re-designcomponent

2. Beta = 1: Random failure mode (useful life)—No life extension possible—complete FMEA.

3. Beta > 2: Wear out failure—Life extension(LE) applicable—exerciseoptionality (LE or retrofit)

Fig. 3 Large rotating planttaxonomy example for sub-components

42 W. Hahn

It can be seen from Fig. 5 that the approach has some rules that need to beapplied as part of the analysis during the failure of a component in association to thebeta component. The rules that need to be applied for assessment of componentsfailure includes: Answers to the following to be incorporated into the beta valueevaluation.

• What is the components original design life? (operating hours)• What was the designed mode of operation? (start -stop or continuous

running)• What was the operational capacity design intent? (components utilised

beyond design)• What degradation mechanisms have been allowed for in the original design?• Is the beta value for this component typical for other operators with similar

conditions?

The hazard rate Z (t), plays a significant role in the wear out phase as the slope ofthe curve needs to be managed by the variables that could influence the curve.

Hazard rate, Z (t)

Time (t)

Phase 1 Phase 2

125000hrs 254000hrs

Phase 3 Phase 4

A

BEnd of the wear out phase & start of life extension.

Fig. 4 Example of the hazard phase plot during the plant lifecycle

β=2

β=4

β=6f(t)

t

β=1

Fig. 5 Probability densityfunction with beta valuesrepresentation

Managing Life Extension for Large Rotating Plant… 43

The importance of differentiation between life extension and wear out lies in thedecision making of how a system, unit, sub-unit or component needs to behave andmaintained up to point of de-commissioning, influencing the slope of the curve asper Fig. 4. During life extension, it is recognised that condition based assessmentwill be incorporated into the major part of the maintenance work and conditionmonitoring during the operating regimes will be adjusted to accommodate theoutcome of the life expectancy It is recognised that the earlier the life extensionperiod start the better the outcome will be for reliability and risk from a holisticasset management point of view.

3 Failure Modes and Monitoring on Rotating Plant

In steam turbines, a number of components need less than ten thousand cycles forlow cycle fatigue related failures to occur. The thermal origin of the low cyclefatigue is a result of the thermal stresses manifesting as a result of thermalexpansion in the turbine. In this case, fatigue results from the cyclic strain ratherthan the stress cycle [3]. The structural changes in the material that occur duringcyclic stress related failures can be divided into three distinct phases, they are:

1. Crack initiation—this includes early development of fatigue damage andcould in certain instances be removed by annealing the materials. Crackpropagation takes place along slip bands, these are normally a few nm percycle.

2. Slip band crack growth—this involves the deepening of the initial microcrack on planes of high shear stress also known as first stage crack growth.The crack propagation is normally a few microns per cycle with visiblestriations per tensile stress cycle.

3. Ultimate ductile failure—this occurs when the crack has propagated to asufficient length so that the remaining cross sectional area can no longersupport the applied load.

Cold end materials in the risk register are examined from a metallurgical andmechanical properties perspective as well as tested in the laboratory to establishendurance fatigue life where applicable.

4 IP Turbine Bore Cracking: Life and ConditionManagement

In 2011, the IP rotor depicted in Fig. 6 was removed after 12 years of operationbetween inspections with the intent of completing a health check, executingremedial work and returning the rotor to service in another unit as part of anexchange program for another 12 years of operation.

44 W. Hahn

The 2011, non destructive testing on the West Burton Unit 4 IP Rotor reported68 indications, most of these were relatively small with radial through-thicknessmeasurements reported to be <3 mm [5]. A number of indications were reported tobe more significant, three of which were highlighted below in Table 1.

Three major defects were found at distances away from the bore centre line asindicated in Fig. 7. These defects had grown from information gathered between the12 year inspection intervals. The forging quality in the 1950s and 1960s was veryvariable, which led to a large number of embedded defects caused by impurities andother constituents in the rotor pencil shafts as can be seen in Fig. 8.

The indications all have different circumferential positions but have been plottedin one circumferential plane to simplify visualization. Figure 8 includes the location

Fig. 6 IP rotor on supports inoff-site works

Table 1 Position of major through wall extent (TWE) flaw indications

Flawidentified

Length(mm)

Depth from Bore(mm)

CircumferentialPosition

Through Wall extent(mm)

BF 75 18–44 212–232 11

AA 43 5 18 18

AJ 37 9 352–364 30

40.00 mm

34.00 mm72.00 mm

9.00 mm 18.00 mm75.00 mm BFAJAA

80.00 mm

Fig. 7 Three major defects locations depicted with relation to the bore position

Managing Life Extension for Large Rotating Plant… 45

of the bore (dashed blue line) with respect to distance from the centre and axialdistance from the steam end [6].

The rotor was bottle bored and all three major defects removed, limiting futurelife to 1,000 starts before the next inspection for underwriting the integrity [5].

5 LP Rotor Pencil Shaft Cracking: Root Cause Analysisand Life Extension Options

The LP rotor depicted in Fig. 9 was removed from service after detecting a crack inthe pencil shaft from vibration monitoring during transient conditions (Start up andshut down vibration signatures). Further confirmation of the cracked rotor wascompleted through a series of truth checks and non destructive testing.

The rotor is of dual flow type and construction is made up of a pencil shaft withshrunk on disc and button drive for each stage with centre collar as per Fig. 10. The

Fig. 8 Embedded defects from the original forging process in the rotor body

Fig. 9 Cracked low pressureEnglish Electric rotor onsupports

46 W. Hahn

location of the collars and discs made the non destructive testing difficult duringcrack detection by virtue of the angles and distances from testing surface with thecrack location in the circled area.

The cracked rotor covered more than 200,000 operating hours and 3,000 startups to point of the large crack detection. The extent of the crack covered approx-imately 270 degrees of the circumference and the measured value from the nondestructive testing indicated a crack depth of approximately 200 mm deep [7].

Further metallurgical and destructive examination was planned at the time on therotor to determine exact root cause and origin of the crack to ensure future lifemanagement mitigation measures are incorporated into the life plans.

Fig. 10 English Electric rotor body with shrunk on discs and blade fixings

Fig. 11 Low pressure turbine with cracked blade removed and rotor in situ

Managing Life Extension for Large Rotating Plant… 47

6 LP Turbine Erosion and Blade Cracking: Life ExtensionThrough Monitoring

As part of life extension and efficiency enhancement on coal fired power stations,retrofitting was completed on Low Pressure turbines and then High Pressure tur-bines. As a result of the aggressive start up regime, blade erosion on the leadingedges and trailing edges of retrofitted LP Turbines cause erosion damage on theblade surfaces [8]. The low pressure turbine last stage blade damage and crackpropagation are mainly starts related with the cold starts delivering most damagedue to the exposure time (Fig. 11).

The trailing edges in this case, develop cracking above the root platform fromthe notched effect of the erosion and propagate through high cycle fatigue as a resultof blade vibration [9] (Fig. 12).

An accumulative pitting damage model was created from research to monitorturbine blade health through on-line rotation counting and in situ blade inspections[8].

7 Monitoring Requirements During Life Extension

For all three case studies covered under large rotating plant in this document, it isunavoidable to ignore monitoring methods in order to prevent catastrophic failure.Cracking in these cases are associated with early warning signs which can be pickedup through vibration monitoring techniques. Methods such as on-line shaft vibra-tion monitoring and blade tip timing are examples of early warning methods;however, there are limitations in each of the methods and the practicality of theapplications. There remains a need in industry, for a health monitoring system thatcovers the complete rotating unit in its entirety, covering bores, shafts, discs andblades.

Fig. 12 Erosion damage on low pressure turbine blade with cracking in the trailing edge

48 W. Hahn

8 Conclusion

The challenges of plant life extension on large rotating plant remain to be a concernas a result of Power plant generators pushing the boundaries of design life envelopeout into the unknown territory of life extension. Operators require a rigorous riskand reliability management plan demonstrating control over potential catastrophicfailures against an aggressive commercial market. Qualitative and quantitativemethods are required to benchmark best practice in health assessments and con-dition monitoring of large rotating plant. Further development is needed in systemhealth monitoring such as vibration and dynamics, to assist power plant generatorswith effective early warning support on large rotating plant in order to preventcatastrophic failure. All examples in this document were pro-actively managedthrough qualitative and quantitative methods, however, further work is needed tosimplify methods and reduce costs effectively.

References

1. Hahn W, Kidd M (2012) Asset management for a coal fired power station in the wear out phase.School of Mechanical, Aerospace and Civil Engineering, University of Manchester, ManchesterUnited Kingdom

2. BS 5760-4:2003, reliability of systems, equipment and components, part 4, guide to thespecification of dependability requirements

3. Gnedenko BV, Ushakov IA (1995) Probabilistic reliability engineering. Wiley, New York4. BS EN ISO 14224:2006, petroleum, petrochemical and gas industries—collection and exchange

of reliability and maintenance data for equipment5. Patterson IR, Zhang X (2012) Remnant life IP turbine rotors, E. on new build and technology,

Ratcliffe on Soar, Nottinghamshire, NG11 0EE6. Skinner D (2011) West Burton Power Station, Ex. unit 2 IP Rotor, IPWn19006 Interim report,

Eon—PES, Hamms Hall, United Kingdom7. Mikkelsen T Siemens, NDT03-2013, automated UT examination of the West Burton Unit 4

LP3 Turbine and the retired LP1 spindle, 70PO036368. Hahn W, Tasker G, Naylor E, Kidd M (2014) Crack initiation in 14 % Cr Low pressure Turbine

Blade Steel, ASME, Journal of Engineering for Gas Turbines and Power, GTP -13-14259. Sinha J, Hahn W, Elbhbah K et al (2012) Vibration investigation for low pressure turbine last

stage blade failure in steam turbines of a power plant, proceedings of the ASME Turbo Expo2012, Copenhagen, Denmark

Managing Life Extension for Large Rotating Plant… 49